1. Field of Invention
The present invention relates to an electric saw device having two saw blades, which realizes a rolling friction between the two saw blades to reduce a frictional force, and uses a key connection between transmission shafts of the saw blades and a power output shaft to improve stability of the connection.
2. Related Art
In an electric saw device having two saw blades, a first saw blade and a second saw blade are driven to rotate simultaneously in opposite directions to saw a work piece. FIG. 1a is a structural view of two saw blades of the prior art. As shown in FIG. 1a, blade heads 42 and 43 of the first and the second saw blades 40 and 41 are in contact with each other. In a cutting process, the blade heads 42 and 43 may generate a high temperature due to the contact friction. Further, the blade heads 42 and 43 of the two saw blades 40 and 41 may collide with each other easily, so the blade heads 42 and 43 will fall off or be broken, which shortens the life of the saw blades 40 and 41.
In the cutting process of the two saw blades 40 and 41 shown in FIG. 1a, the first and the second saw blades 40 and 41 may be slightly softened due to the heat generated in the friction. Further, the first and the second saw blades 40 and 41 may expand slightly due to the resistance in the cutting, and a distance between the blade heads 42 and 43 of the first and the second saw blades 40 and 41 will increase slightly, so a cut piece 44 may be clamped between the first and the second saw blades 40 and 41 easily, as shown in FIG. 1b. Meanwhile, a width of the cutting line of the saw blades 40 and 41 will increase, which influences the cutting precision. When the distance between the blade heads 42 and 43 of the first and the second saw blades 40 and 41 becomes greater, the problem of the cut piece between the saw blades will become more serious, which may eventually make a motor become useless because the motor is burned due to overload.
FIG. 2a shows a structure for a pair of saw blades, which solves the problems that the cut piece 44 may be clamped between the saw blades 40 and 41 easily shown in FIGS. 1a and 1b and that the blade heads 42 and 43 may be damaged easily. As shown in FIG. 2a, in the structure for a pair of saw blades, a circular rib 55 is disposed on a second saw blade 51, which urges against an inner surface 54 of a first saw blade 50, so as to limit the distance between the two saw blades 50 and 51. The rib 55 keeps a safe distance (0.03 mm) that allows normal operation between blade heads 52 and 53 of the two saw blades 50 and 51. Further, the rib 55 prevents the cut piece from being clamped between the two saw blades 50 and 51.
When the two saw blade 50 and 51 perform the cutting, heat is generated because of the friction between the rib 55 and the first saw blade 50, such that the first saw blade 50 is softened, and the cutting precision is lowered. Further, after a period of use, the rib 55 will be worn (as shown in FIG. 2b), or the first saw blade 50 will be worn (as shown in FIG. 2c). When the worn size is equal to or greater than the distance (0.03 mm) between the blade heads 52 and 53 of the two saw blades 50 and 51, the blade heads 52 and 53 may collide with each other, such that the electric saw device cannot operate normally. At this time, the saw blades 50 and 51 must be replaced.
Referring to FIG. 3, the first saw blade 50 of the electric saw device is driven by a small transmission shaft 60, which is screwed to a power output shaft 61. Further, a fixing screw 62 is screwed to the small transmission shaft 61, so as to tightly fix first the saw blade 50. Meanwhile, the second saw blade 51 is fixed to a large transmission shaft 63.
The power output shaft 61 is tightly connected to the small transmission shaft 60 through threads, so a tilt is inevitable between the small transmission shaft 60 and the power output shaft 61, which makes the saw blades 50 and 51 wiggle, jump, or even damage the saw teeth after assembly. The fixing screw 62 has only 1.5 threads, which are too few. Besides, as the fixing is based on the threads, a secondary tilt may occur. Therefore, the problems that the saw blades 50 and 51 wiggle, jump, or damage the saw teeth become more serious, thus resulting in an unstable current and overload of the electric motor, and shortening the service life of the electric motor.
A locking direction of the small transmission shaft 60 makes the small transmission shaft 60 become loose in operation. Therefore, a super glue must be applied at the connection of the power output shaft 61 and the small transmission shaft 60 to fix them. The results are as follows.
1. The small transmission shaft 60 and the power output shaft 61 cannot be disconnected. The entire machine head must be replaced when some parts are damaged and need to be repaired.
2. When the saw blades 50 and 51 are disassembled and replaced, if the torque to loosen the fixing screw 62 is greater than 30 N, the connection fixed by the super glue may become loose or even break under the torque, so the small transmission shaft 60 and the saw blade 50 get loose at the same time.